1. Field of the Invention
The present invention relates to a multi-frequency antenna with dual loops. More particularly, the present invention relates to a multi-frequency antenna which can operate in two different frequency bands with the dual loops thereof.
2. Description of Related Art
The personal mobile communications technology has already proven its huge potential and business opportunity in the wireless communications industry. In the course of advancement, various systems adopting different technologies and frequency bands have been developed and used in different areas and markets. However, this also brings troubles and inconvenience to system suppliers and consumers due to different systems, such as GSM900, DCS1800, and PCS1900, adopting different frequencies.
In order to bring convenience to the users, people in this field have exerted a lot of efforts in the development of a multi-frequency mobile phone. However, the problem to be solved firstly is the antenna which is considered to be the start as well as the end of the wireless communications, and the following requirements must be satisfied:                1. Frequency and bandwidth; and        2. Radiation pattern and polarization.        
Moreover, it is the tend for the design of electronic products including the mobile phone to become lighter, thinner, shorter, and smaller, even the design of the antenna of mobile phone is influenced. Thus, the conventional planar inverted-F antenna (PIFA) cannot meet the requirements of larger bandwidth gradually. U.S. Pat. No. 6,943,730 discloses one of the multi-frequency and low-profile, capacitively loaded magnetic dipole (CLMD) antennas. Referring to FIG. 1, the antenna 10 uses two top plates 12, 14 and a bottom plate 16 connected with a feed line to create the inductive part 20, 22, so as to be compatible with the low-frequency GSM channel and the high-frequency PCS channel. As disclosed in the specification, in order to broaden the bandwidth, more than two top plates must be used together to increase the multi-frequency effect. Therefore, the structure is not suitable for a compact device with a limited space for accommodating the antenna.
Another antenna that realizes the multi-frequency operation is shown in FIG. 2. The antenna includes a first radiation portion A, a second radiation portion B, and a ground portion C. The first radiation portion A and the second radiation portion B respectively extend from the two opposite portions of the same end of the ground portion C. The first radiation portion A includes a first conductive tab A1 parallel to the ground portion C and a first connection portion A2 connecting the first conductive tab A1 and the ground portion C. The second radiation portion B includes a second conductive tab B1 parallel to the ground portion C and a second connection portion B2 connecting the second conductive tab B1 and the second connection portion B2. The first conductive tab A1 and the second conductive tab B1 respectively extend from the first connection portion A2 and the second connection portion B2 in the same direction.
The above antenna makes the multi-frequency operation possible, but still has the following disadvantages. The first connection portion A2 is excessively close to the second connection portion B2, which does not meet the requirement of the high-frequency bandwidth. Meanwhile, the first connection portion A2 is excessively close to the second connection portion B2, and the first conductive tab A1 and the second conductive tab B1 respectively extend from the first connection portion A2 and the second connection portion in the same direction, so the fabrication is difficult when bending the first radiation portion A and the second radiation portion B and when welding the feed line onto the first conductive tab.
The present invention provides a solution to the above problems, which can significantly broaden the multi-frequency high-frequency bandwidth and simplify the fabricating process of the antenna.